1. Field of the Invention
The present invention relates to a switching power converter and, more specifically, to dynamically driving the switching transistor of a switching power converter.
2. Description of the Related Arts
Switching power converters include a power stage for delivering electrical power from a power source to a load, a switching device in the power stage that electrically couples or decouples the load to or from the power source, and a switch controller coupled to the switch for controlling the on-times and off-times of the switching device. The switch is typically a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) or a BJT (Bipolar Junction Transistor). A switch controller includes a pulse generator which generates a pulse for driving the switching transistor on or off. For example, the logic high and logic low parts of the pulse correspond to the on-times and off-times of the switching transistor. That is, the switching transistor is turned on while the pulse is in its logic high state and the switching transistor is turned off while the pulse is in its logic low state. The on-times and off-times of the switch can be modified by the switch controller based upon a feedback signal representing the output power, output voltage or output current.
FIG. 1A conceptually illustrates a conventional flyback type switching AC-DC power converter using a BJT 108 as the switching device. The power converter includes a transformer 110, a diode D1, a capacitor CO, a BJT switch 108, a pulse generator 102, and a switch drive module 106. The pulse generator 110 generates the output drive signal 114 that drives the switch 108. The switch drive module 106 includes switches 112, 114. The switch drive module 106 shifts the voltage level of the logic level output drive pulses 104 to generate a switch drive signal 107 with a high voltage (+Vcc) suitable for driving the switch 106 on, when the output drive pulse 114 is at its logic high state, by turning the switch 112 on and the switch 114 off. The switch drive module 106 shifts the voltage level of the logic level output drive pulses 104 to generate a switch drive signal 107 with a low voltage (−Vdd) suitable for turning off the switch 106, when the output drive pulse 114 is at its low state, by turning the switch 112 off and the switch 114 on.
The rectified input AC power (DC) is stored in the transformer 110 while the switch 108 is turned on because the diode D1 becomes reverse biased. The rectified input AC power (DC) is then transferred to the load (not shown) across the capacitor CO while the switch 108 is turned off because the diode D1 becomes forward biased. Pulse Width Modulation (PWM) and Pulse Frequency Modulation (PFM) are conventional techniques used for controlling the switching power converters by controlling the widths or frequencies of the output drive pulse 114 driving the switch 108 to achieve output power regulation. For example, the width of the output drive pulse 114 in the PWM control scheme is regulated by the pulse generator 102 to achieve load and line regulation and output voltage ripple regulation.
As mentioned above, the switching device in the switching power converter may be a MOSFET or a BJT, which is typically a silicon semiconductor device. When the switching device is a MOSFET, the pulse generator can directly apply the +Vcc voltage to the gate of the MOSFET during the on-time, and the gate of the MOSFET is short to ground or −Vdd during the off-time. Such output drive is relative simple.
However, when the switching device is a BJT 108 as shown in FIG. 1, it is undesirable to apply the +Vcc directly to the base of the BJT 108, since the BJT 108 is a current-drive device. The base-emitter junction of the BJT 108 should be forward biased by approximately 0.6 V in order for the BJT 108 to function properly. One of the important properties of the BJT 108 is that its small base current IB controls the amount of the much larger collector current IC, i.e., IC=β×IB, where β is a current amplification factor dependent on the physical properties of the BJT 108. Typical values of the β range for a power BJT is from 5 to 50.
The BJT switching device 108 typically has major turn-on and turn-off delay. The BJT has long turn-off time due to its long storage time. Normally, if
            I      B        <                  I        C            β        ,the BJT 108 operates in the active range, or the BJT 108 cannot fully turn on. If
            I      B        >                  I        C            β        ,the BJT 108 operates in the deep saturation range, and the turn-off time is longer. Ideally, the base drive current IB should be proportional to the collector current IC.
FIG. 1B illustrates an ideal relationship between the base current and the collector current of a BJT switch in a switching power converter. In a flyback type switching power converter, the collector current IC has a positive slop 120. The magnitude of the positive slope 120 varies with the output load and input voltage conditions. Note that if the base drive current IB is too high, the turn-on will be very fast, but it causes high initial turn-on current spike 118. This spike 118 generates harmonic current, and causes EMI (Electro-Magnetic Interference).
Therefore, what is needed is a technique for effectively controlling the output drive (current or voltage) of a switching device of a switching power converter to reduce the turn-on and turn-off time of the switching device and control initial current spike. There is also a need for a technique for effectively controlling the output drive current of a BJT switching device so that a proportional relation between the base current and the collector current of the BJT switching device is maintained in a variety of operation conditions.